High efficiency double heterostructure bipolar transistor devices are frequently made with the junctions clad by thin graded quaternary regions. In this structure the junctions are buried for improved device reliability. In the manufacture of these devices, the multilayer stack is first grown, then electrical contacts to the buried base and collector are made by forming mesas to give physical access to the base and collector layers. The contact to the base emitter junction is formed by diffusion of the contact metal through the thin graded quaternary layer between the base and emitter layers. Because the diffusion process is subject to so many variables, low resistance contacts to the base are difficult to form without punch-through to the collector layer. Efforts to avoid punch-through to the collector may result in too shallow diffusion of the metal contact material, with a resultant high resistance or non-ohmic contact. Therefore control of the diffusion depth of the metal contact layer is critical.